The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the reduction of image artifacts caused by patient motion during an MRI scan by determining a scan prescription using physiological information.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9cdippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Typical magnetic resonance exams can be divided into two broad stages: localization and information gathering. In the information gathering stage, specific scan prescriptions are utilized to obtain the required clinical information. The localization stage is generally less well-defined and is needed simply to determine the appropriate location and timing of the subsequent information gathering stage. In areas where there is significant motion due to cardiac contractions and respiration, this timing and localization is critical as the area of interest may move out of the prescribed scan location in portions of the cardiac or respiratory cycles. Also, motion during the acquisition of data can cause significant artifacts in the resultant MR images.
The challenges of scanning in and around the heart in the presence of cardiac and respiratory motion are well known. For 2D scanning techniques it is essential to localize not just to a particular spatial location, but also to an exact cardiac and respiratory phase as the structures of interest may move out of the scan plane during either physiological cycle. For 3D imaging it is advantageous to have structures of interest in-plane since through-plane resolution is typically poorer. In addition, due to the relatively long scan times required for 3D vs. 2D scans, it is advantageous to scan for as long a portion of the cardiac cycle as possible for greater scan efficiency. Erroneous timing can result in significant motion during the acquisition resulting in a degradation of image quality.
Conventional magnetic resonance imaging in and around the heart has often used some form of cardiac and respiratory compensation or gating to synthesize an image at specific phases of the cardiac and/or respiratory cycles from data acquired over multiple heart beats and respiratory cycles. This methodology not only has the potential for error due to inconsistencies between cycles, but can take a long time and only produces images for a limited number of spatial locations and points within either the cardiac or respiratory cycles.
Realtime or fluoroscopic MRI is a technique that allows imaging to occur at rates up to 20-30 frames per second. In certain situations, realtime MRI has the potential to be used for both the localization and the information gathering stages. However, in many applications a subsequent imaging series is still desired. In this situation realtime MRI has the potential to provide rapid localization of the desired spatial position in any arbitrary scan plan and this localization information can be used in the subsequent scan.
Acquiring magnetic resonance (MR) images may require a time period of seconds to minutes. Over this period, significant anatomical motion may occurxe2x80x94specifically, cardiac- and respiratory-induced motion. This motion produces artifacts that may significantly degrade image quality. A number of different techniques have been developed in order to compensate for the effects of this motion. These techniques attempt to either acquire data during periods of minimal motion, or to correct for the effects of motion when it does occur. For these techniques to compensate for the effects of motion, the motion itself must be known accurately throughout the data acquisition. In the past, bellows and navigator echoes placed on the diaphragm have been used to determine respiratory-induced motion. A shortcoming of this approach is that the relationship between diaphragm position and respiratory-induced motion at anatomy remote from the diaphragm will vary from patient to patient and across studies. For cardiac-induced motion, ECG-waveforms have been used. The problem with ECG waveforms is that there may be substantial variation across patients and across studies. A drawback with some previous motion compensation techniques is that they assume that one patient""s anatomy is best viewed at the same part of the cardiac cycle (or respiratory cycle) as was used to view the same feature on the previous patient.
The present method and system identifies the proper position and timing to use for a magnetic resonance scan based on a previous initial realtime acquisition where anatomy, the point in the cardiac cycle and the point in the respiratory cycle are sampled many times per second. The gating times and acquisition durations are tailored to match a particular patient during a single patient session (i.e., the patient initial realtime acquisition of MR data and a further MRI are performed without the patient leaving). The relative position in the cardiac and respiratory cycles, using any form of cardiac and respiratory monitoring tools, is associated with each frame in an initial realtime MR acquisition. Such monitoring tools could include, but would not be limited to, peripheral plethymographs or ECG leads; pneumatic bellows or MR position measurements (e.g. navigator echoes).
Once the specific cardiac and respiratory phase are associated with each image in a realtime MR data set, a rapid analysis on the realtime image set is performed to determine initial times and durations over which certain specified criteria are satisfied. The initial times with the cardiac and respiratory cycles will be used to set initial timing values in the subsequent MR scan and the determined duration will be used to guide the choice of parameters which dictate the length over which image data is acquired.
Importantly, cardiac and respiratory information are collected in conjunction with realtime MR imaging and the physiological data is used in conjunction with the anatomic or spatial information to identify the correct gating times in the cardiac and respiratory cycles as well as the optimal acquisition durations with each of these cycles.